Several techniques and systems have been developed for correcting and stabilizing injuries to or malformation of the spine. In one type of system, an elongated member such as an elongate bendable rod is disposed longitudinally along a length of the spine, spanning two or more vertebral levels. In certain applications, the rod is bent to correspond to the normal curvature of the spine in the particular region being instrumented, such as the normal kyphotic curvature of the thoracic region or the lordotic curvature of the lumbar region. In accordance with such a system, the rod is engaged to various vertebrae along a length of the spinal column by way of a number of anchor devices that utilize a variety of fixation elements configured to engage specific portions of the vertebra and other bones. For instance, one such fixation element is a hook that is configured to engage the laminae of the vertebra. Another very prevalent fixation element is a bone screw that can be threaded into various parts of the vertebrae, particularly the pedicle.
As these systems have evolved, various degrees of freedom of relative orientation were integrated into the systems in order to accommodate misaligned spinal curvature as well as to minimize rod bending and stress risers in the anchor. Presently, the standard in rod-based spinal systems is a polyaxial pedicle screw which includes a yoke having a slot for receiving a bendable elongate connecting rod that is coupled to a threaded bone engaging shank for polyaxial swiveling movement with respect thereto. One example of such a polyaxial screw is shown and described in commonly assigned U.S. Pat. No. 8,491,639, entitled “Multi-axial Spinal Fixation System”, issued on Apr. 24, 2012 to Anthony James et al (the '639 patent).
One particular use of polyaxial pedicle screws is for posterior application in the cervico-thoracic spine. In such application, due in part to the relatively small size of the vertebrae and access area for surgery, low profile and high angulation of the polyaxial screws are desirable. While considering these desirable features, some have recognized that pull out strength of the polyaxial screw may be affected in an effort to provide a structure to achieve such results. This concern is described, for example, in U.S. Pat. No. 8,636,778, issued to Gephart et al. on Jan. 28, 2014. Other patents describing polyaxial screws that may be used in cervico-thoracic applications include U.S. Pat. No. 8,337,530, issued to Hestad et al. on Dec. 25, 2012, U.S. Pat. No. 8,100,946, issued to Strausbaugh et al. on Jan. 24, 2012 and U.S. Pat. No. 8,021,397, issued to Farris et al. on Sep. 20, 2011.
In addition, after placement of the polyaxial screw during such spinal surgery, it is also desirable that the yoke of the polyaxial screw remain upright. Screw yokes are often manipulated with a positioner instrument for later rod placement and for a preview of any rod contouring that may be necessary. Yokes that stay in position negate the need for repositioning and therefore facilitate a more efficient operation. The '639 patent noted hereinabove describes use of a wave spring to provide friction between the yoke and the bone screw to releasably retain the yoke in position after screw placement.
Nevertheless, improvements in polyaxial screws that provide relatively high angulation and low profile, as well as retention of the screw yoke in a releasable manipulable position once introduced into a vertebra, particularly for application in the posterior cervico-thoracic spine, are desirable.